Geodesic
Covering elements in the lattice of varieties of algebras
Matematičeskie zametki, Tome 15 (1974) no. 2, pp. 307-312.

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Let variety $\mathfrak U$ be given by the balanced identities of signature $\Omega$ not containing unary operations. Then, in the lattice of subvarieties of variety $\mathfrak U$, any element different from $\mathfrak U$ has an element covering it. In particular, variety $\mathfrak U$ might be the varieties of semigroups, groupoids, $n$-associatives, etc. It is also proven that, in the lattice of varieties of semigroups, there exists an element having a continuum of covering elements. 
@article{MZM_1974_15_2_a16,
     author = {A. N. Trakhtman},
     title = {Covering elements in the lattice of varieties of algebras},
     journal = {Matemati\v{c}eskie zametki},
     pages = {307--312},
     publisher = {mathdoc},
     volume = {15},
     number = {2},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1974_15_2_a16/}
}
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A. N. Trakhtman. Covering elements in the lattice of varieties of algebras. Matematičeskie zametki, Tome 15 (1974) no. 2, pp. 307-312. http://geodesic.mathdoc.fr/item/MZM_1974_15_2_a16/